We plan to continue our studies on the arrangement of the cross bridges of muscle thick filaments in the resting and rigor state using bifunctional reagents to detect configurational changes of the cross bridges in the presence and absence of various ligands. Glycerinated myofibrils will be treated with various bifunctional reagents in the presence of metabolites important in contraction (e.g., MgATP, MgADP, Ca 2 ion and the rate of cross linking the myosin heads to the thick filament surface will be followed by measuring the relative amounts of SF1 and myosin rod released on treatment of the myofibril by papain and chymotrypsin. These studies are relevant to the movement and orientation of the myosin cross bridges when muscle is stimulated to contract. Our recent studies demonstrate that reconstituted actomyosin threads formed from highly polymerized vertebrate F-actin and containing only trace amounts of myosin show rapid shrinkage on addition of MgATP. We hope to determine the minimum size of the myosin species (monomer, dimer, bipolar filament) which is required for contraction in these systems by cross linking the species within the threads at various levels of contraction. The role of the tail segment of myosin in the contractile mechanism from mixtures of native myosin and heavy meromyosin. These studies are relevant to the mechanism of motile processes in non-muscle cells. BIBLIOGRAPHIC REFERENCES: Harrington, W.F., Reisler, E., and Burke, M. An Activation Mechanism for ATP Cleavage in Muscle. J. of Supramolecular Structure 3, 112 (1975). Burke, M., Reisler, E., and Harrington, W.F. The Effect of Bridging the Two Essential Thiols of Myosin on its Spectral and Actin Binding Properties. Biochemistry 15, 1923 (1976).